Joining or repair of superalloy articles for high temperature applications can be achieved using welding or brazing. For superalloys containing substantial amounts of gamma prime or gamma double prime, welding can cause excessive cracking in the heat affected zone and fusion zone in addition to distortion of the article. Brazing, on the other hand, has advantages over welding in terms of capability in joining hard-to-weld superalloy articles and in reducing cost due to its suitable nature for batch processing. During a brazing cycle, the braze alloy melts, joins the superalloys, and solidifies either during cooling or isothermally via diffusion process. The superalloy articles to be joined must normally remain in solid state during brazing, and as such the braze alloys must have a much lower solidus (melting temperature) than the superalloys.
The following US Patents and Patent applications are related to this subject:
4,073,639February 1978Duvall, et al.4,381,944May, 1983Smith, Jr.,.4,830,934May 1989Ferrigno, et al.5,240,491August 1993Budinger et al.5,735,448April 1998Draghi et al.5,783,318July 1998Biondo et al.6,520,401February 2003Miglietti6,190,467Feb.20, 2001Jackson et al.6,982,123January 2006Budinger, et al.7,156,280January 2007Jiang et al.7,261,758August 2007Okada, et al.7,279,229October 2007Budinger, et al.2003/0002988January 2003Jackson, et al.2004/0184945September 2004Sjodin, et al.2005/0067061March 2005Huang, et al.2006/0068214March 2006Gigliotti, et al.
Traditional nickel-based braze alloys contain relatively high amounts of boron and silicon (and phosphorus) in order to reduce the melting temperature of a nickel or nickel-chromium matrix down to suitable brazing temperatures of between 1000 degree C. to 1250 degree C. (America Welding Society (AWS): Brazing Handbook, 4th ed., 1991). Furthermore, these elements are responsible for the wetting and flow behaviors of the braze alloys on the superalloy substrates. The use of boron or/and silicon as melting depressants in low melting braze alloys with or without the addition of high-melting filler alloy and related processes are described in U.S. Pat. Nos. 4,073,639; 4,381,944; 4,830,934; 5,735,448; 6,982,123; 7,261,758 and 7,279,229 listed above.
Boron, due to its high diffusivity, is preferred where homogeneous joint compositions are required. However, a costly diffusion process is needed in order to improve the mechanical integrity of the brazed joint/repaired area as boron forms brittle hard phases with other alloying elements within the joint/repaired area, reducing the ductility, fatigue life and corrosion resistance of the joint/repaired area. In addition to the cost associated with such diffusion heat treatments, prolonged heat treatments can also compromise the properties of the superalloy articles as a result of microstructural changes.
As such, the search for alternative melting point depressants while ensuring adequate ductility is needed. The use of hafnium as a melting point depressant for nickel based braze alloys represents a new approach to producing ductile braze alloys with moderate brazing temperatures below 1240 degree C.
U.S. Pat. No. 7,156,280 listed above uses hafnium as a melting point depressant to reduce the total amount of boron in the braze alloy. However, in that disclosure boron is also included in the braze alloy compositions claimed.
Ductile braze alloys containing nickel-hafnium-chromium, nickel-hafnium-cobalt and nickel-hafnium-molybdenum were proposed by Buschke and Lugscheider (Proceedings from Materials Conference '98 on Joining of Advanced and Specialty Materials, 12-15 Oct. 1998, Illinois) as an alternative braze alloy to boron and silicon containing braze alloys. While these alloys show good ductility, they require both very high brazing temperatures (1235 degree C.) in order to produce good wetting as well as a long diffusion cycle to homogenize the joints. Additionally, joints with nickel-hafnium-chromium suffer from galvanic corrosion when tested in aqueous salt solution (Humm and Lugscheide, Proceedings from Joining of Advanced and Specialty Materials, 5-8 Nov. 2001, Indianapolis, Ind., ASM International, 2002).
An addition of other elements to the nickel-hafnium alloys was also reported. This alloy contains Ni, 18.6% cobalt by weight, 4.5% chromium by weight, 4.7% tungsten by weight, and 25.6% hafnium by weight (Zheng, et al. Acta Met. Sinica, Vol. 3, No. 5, 1990 335-340). The addition of tungsten in the alloy contributed to an increase in the melting temperature of the alloy. As such, this alloy also requires an elevated brazing temperature. Additionally, due to the amount of hafnium in the braze alloy, the joint composition greatly deviates from that of the superalloy articles.
Where hafnium is present in a low-weight percentage, it generally acts as a grain boundary strengthener, not as a melting point depressant. U.S. Pat. No. 5,783,318 listed above claims a nickel-based welding filler metal with addition of 0.03-2.5% hafnium by weight, 0.003-0.32% boron by weight, and 0.007-0.35% zirconium by weight. The addition of hafnium, boron and zirconium is required for grain boundary strengthening when repairing single crystal superalloy articles. However, the weight percentage of the addition of these elements is too low to depress the melting temperature of the welding filler alloy.
In addition to the use of hafnium as a melting point depressant, zirconium exhibits similar function. U.S. Pat. No. 6,190,467 listed above discloses braze alloys with zirconium and boron as primary melting point depressants. Another nickel-based alloy containing 10.4% cobalt by weight, 8.5% chromium by weight, 4.4% tungsten by weight, and 13.4% zirconium by weight requires an excessively high brazing temperature of 1270 degree C. to achieve good joints (Zheng, et al. Journal of Materials Science, 28 (1993) 823-829).
U.S. Pat. No. 6,520,401 listed above describes nickel based boron-free braze alloy containing either 26-34% hafnium by weight or 11-19% or 40-60% zirconium by weight. The intermetallic phases formed in the nickel-zirconium eutectic alloys are found to be softer than borides. However, the addition of hafnium or zirconium is excessive thus preventing the brazed joint from reaching compositions similar to a superalloy substrate even with the addition of filler alloys. Additionally, in that invention with the addition of hafnium or zirconium alone in nickel, it states that the lowest achievable melting temperatures are 1190 and 1170 degree C., respectively. A brazing temperature range from 1230 degree C. to 1320 degree C. for up to 36 hours is required. As such, braze alloy systems containing either hafnium or zirconium or are not able to achieve melting temperatures approaching that of boron-containing braze alloys. It is therefore necessary to find other alloying elements which can be incorporated in the nickel-hafnium, nickel-zirconium and nickel-hafnium-zirconium alloy systems, and the content of hafnium needs to be reduced to enable a braze joint to approach the compositions of the superalloy articles.
A nickel based braze alloy with manganese addition was also reported for epitaxial brazing of single crystal superalloys (B. Laux, S. Piegert and J. Rosler, TurboExpo 2008: Power for Land, Sea and Air, Jun. 9-13, 2008, Berlin, Germany, paper number GT2008-50055). This alloy, however, contains excessive manganese ranging from 20% to 58% by weight. Manganese is normally considered an incidental element in superalloys, and most superalloy specifications call for control of manganese to be less than 1% by weight.
U.S. Pat. No. 5,240,491 listed above discloses a braze alloy mixture containing two or more alloy powder compositions, one of the which is high-melting filler alloy selected based on the superalloy articles to be repaired/joined, and another is a low-melting braze alloy containing boron or/and silicon as melting point depressant. A third powder, termed eutectic alloy (3.1-8.2% cobalt by weight, 6.8-38.5% chromium by weight, 0-12.6% aluminum by weight, 0-11.5% titanium by weight, 0-1.3% molybdenum by weight, 0-23.1% tantalum by weight, 0-2.4% tungsten by weight, 0-5.1% niobium by weight, 0-1% rhenium by weight, 0-0.4% hafnium by weight, and 0-0.6% yttrium by weight), is formulated based on eutectic compositions in the superalloys and is used to assist liquid flow characteristics and alloying of the mixture. The eutectic alloy may become substantially liquid during brazing at 1260 degree C. and is not used as low-melting braze alloy.
While a few boron-free braze alloys have been formulated to join and repair nickel based superalloy articles, the prior art lacks boron-free nickel-based braze alloy compositions composed of multiple melting point depressants, each in moderate amount, to reduce the melting temperature and diffusion time and to provide joints with identical compositions as the substrate superalloys using matching filler alloys.